Fitting of a hearing aid to a particular person's hearing impairment generally requires knowledge of clinically measured hearing thresholds for the person in question. The auditory brainstem response (ABR) can be used as an objective estimate of audiometric hearing thresholds (e.g. [Stürzebecher et al., 2006]). ABR signals are traditionally measured by surface electrodes mounted on the head with one electrode at the vertex or in the middle of the forehead, one behind the ear on the mastoid or on the earlobe, and one ground electrode on the opposite side of the head. Future hearing aids may, however, include electrodes on the surface of the hearing aid shell facing the ear canal to record electric brain wave signals such as an electroencephalogram (EEG) (cf. e.g. [Lunner, 2010]).
A portable EEG monitoring apparatus is described in [Kidmose and Westermann, 2010]. A hearing aid comprising electrodes for detecting electrical signals such as brain waves is described in [Kidmose and Mandic, 2011]. The design of stimuli for a system for the recordal of an auditory brainstem response (ABR) of a person is e.g. described in WO 2006/003172 A1.